Pharmacological characterization of neurogenic responses of the sheep isolated internal anal sphincter

Dual effects of α2 -adrenoceptors in modulating myogenic tone in sheep isolated internal anal sphincter

BACKGROUND

The role of α-adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of α1 -adrenoceptors. We have used three clinically available agents, which are selective for α2 -adrenoceptors, to investigate their role in contractile and neurogenic responses on the internal anal sphincter (IAS).

METHODS

IAS strips, which had spontaneously generated tone, were used to investigate the contractile effect of lofexidine, brimonidine, and dexmedetomidine on muscle tone in the presence or absence of subtype selective antagonists. The effect of brimonidine on the magnitude and time course of neurogenic responses generated by electrical field stimulation (EFS) was also examined. The affinity of test compounds at α1 - and α2 -adrenoceptors was established by competition binding with [3H]-prazosin and [3H]-RX821002.

KEY RESULTS

All agonists caused concentration-dependent contraction of the IAS and lofexidine demonstrated an enantiomeric difference in potency with a 10-fold difference between the (-) and (+) isomers. Responses to lofexidine and dexmedetomidine were inhibited in the presence of the α1 -adrenoceptor selective antagonist prazosin, but not in the presence of RX811059 (α2 -adrenoceptor selective antagonist); brimonidine responses were inhibited by RX811059 and, to a lesser extent, by prazosin. Brimonidine affected both magnitude and duration of neurogenic responses, which was reversed in the presence of RX811059.

CONCLUSIONS & INFERENCES

We conclude that α2 -adrenoceptors can mediate contraction of IAS, although this effect is most evident with efficacious imidazoline agonists rather than the most selective ligand. In addition, this receptor subtype can directly inhibit noradrenergic contractile responses to EFS and, indirectly, enhance nitrergic relaxatory responses.

Pharmacological characteristics of endothelin receptors on sheep rectal blood vessels

Haemorrhoids is associated with high blood flow of the anorectal region. The question of whether pharmacological manipulation of vascular supply can relieve the symptoms of haemorrhoids has been raised. In order to undertake this type of clinical investigation, it is first essential to gain a better understanding of the properties of vascular receptors that may regulate blood flow into anal cushions and haemorrhoids. Due to the limited availability of human anorectal specimens and the good reliability of sheep tissue as an experimental model of human anorectal diseases, we studied the properties of endothelin receptors in sheep rectal artery (SRA) and vein (SRV), the vessels contributing to the blood flow of haemorrhoidal plexus, using isometric tension recordings. We found that endothelin-1 and sarafotoxin 6a were very potent constrictor agents in both SRA and SRV. The selective ET(A) receptor antagonist PD156707 (100 nM) produced a parallel rightward displacement of ET-1-induced contractions in both vessels and abolished sarafotoxin 6a-induced contractions in the SRA. PD156707 (3 μM) practically abolished contractions to ET-1 in the SRA, suggesting that the response is entirely mediated by ET(A) receptors. While, the selective ET(B) receptor antagonist BQ788 (100 nM) caused no significant change in ET-1-induced contractions in both vessels, a minor role for ET(B) receptor subtype to responses to sarafotoxin 6a in the artery was suggested.

Investigation of the distribution and function of alpha-adrenoceptors in the sheep isolated internal anal sphincter

BACKGROUND AND PURPOSE

We have investigated the distribution of alpha-adrenoceptors in sheep internal anal sphincter (IAS), as a model for the human tissue, and evaluated various imidazoline derivatives for potential treatment of faecal incontinence.

EXPERIMENTAL APPROACH

Saturation and competition binding with (3)H-prazosin and (3)H-RX821002 were used to confirm the presence and density of alpha-adrenoceptors in sheep IAS, and the affinity of imidazoline compounds at these receptors. A combination of in vitro receptor autoradiography and immunohistochemistry was used to investigate the regional distribution of binding sites. Contractile activity of imidazoline-based compounds on sheep IAS was assessed by isometric tension recording.

KEY RESULTS

Saturation binding confirmed the presence of both alpha(1)- and alpha(2)-adrenoceptors, and subsequent characterization with sub-type-selective agents, identified them as alpha(1A)- and alpha(2D)-adrenoceptor sub-types. Autoradiographic studies with (3)H-prazosin showed a positive association of alpha(1)-adrenoceptors with immunohistochemically identified smooth muscle fibres. Anti-alpha(1)-adrenoceptor immunohistochemistry revealed similar distributions of the receptor in sheep and human IAS. The imidazoline compounds caused concentration-dependent contractions of the anal sphincter, but the maximum responses were less than those elicited by l-erythro-methoxamine, a standard non-imidazoline alpha(1)-adrenoceptor agonist. Prazosin (selective alpha(1)-adrenoceptor antagonist) significantly reduced the magnitude of contraction to l-erythro-methoxamine at the highest concentration used. Both prazosin and RX811059 (a selective alpha(2)-adrenoceptor antagonist) reduced the potency (pEC(50)) of clonidine.

CONCLUSIONS AND IMPLICATIONS

This study shows that both alpha(1)- and alpha(2)-adrenoceptors are expressed in the sheep IAS, and contribute (perhaps synergistically) to contractions elicited by various imidazoline derivatives. These agents may prove useful in the treatment of faecal incontinence.

A comparative study of structure and function of the longitudinal muscle of the anal canal and the internal anal sphincter in pigs

PURPOSE

This study aims to compare the physiology of the longitudinal muscle of the anal canal with the internal anal sphincter in pigs.

METHODS

Histology and in vitro studies were performed to compare the effect of neural responses induced by electric stimulation and through nicotinic, purinergic, and serotoninergic receptors.

RESULTS

The longitudinal muscle and the internal anal sphincter are constituted exclusively by smooth muscle. Strips from the internal anal sphincter a) developed myogenic tone; b) responded to electric stimulation with an "on" relaxation antagonized by nitric oxide synthase inhibitors and purinergic P2Y1 antagonists, and with an "off" contraction antagonized by atropine and phentolamine; and c) responded to stimulation of nicotinic receptors with a relaxation antagonized by nitrergic and purinergic P2Y1 antagonists, responded to stimulation of serotoninergic 5-hydroxytryptamine 3 receptors with a contraction, and relaxed to carbachol and purinergic P2X agonists. Strips from the longitudinal muscle a) did not develop tone, b) responded to electric stimulation with an "on" contraction antagonized by atropine, and c) did not respond to stimulation of nicotinic or serotoninergic 5-hydroxytryptamine 3 receptors, and carbachol and purinergic P2X agonists induced a contraction.

CONCLUSIONS

The motility of the internal anal sphincter includes myogenic tone, relaxation mediated by nitric oxide and purinergic P2Y1 receptors, and contraction mediated by cholinergic motor neurons and sympathetic fibers. The motility of the longitudinal muscle is limited to a contraction mediated by cholinergic neurons, suggesting that longitudinal muscle contracts during relaxation of the internal sphincter, shortening the anal canal. Nicotinic, muscarinic, and serotoninergic receptors might be therapeutic targets for anal motor disorders.

Pharmacology of the internal anal sphincter and its relevance to faecal incontinence

1 The internal anal sphincter (IAS) has a spontaneous tone and is the main contributor to the maintenance of faecal continence. The spontaneous resting tone exhibited by the sphincter can be modified by neurotransmitters from the autonomic and enteric nervous systems. 2 In this review, the influence of the sympathetic and parasympathetic nervous systems on IAS tone are discussed and the putative roles of nitric oxide, carbon monoxide, vasoactive intestinal peptide and adenosine triphosphate in non-adrenergic non-cholinergic transmission are considered. 3 Faecal incontinence is a common condition that places a heavy financial burden on the health service and severely affects patients' quality of life. Resting anal pressure is reduced in patients with faecal incontinence and agents that increase sphincter tone tend to relieve symptoms. The results of clinical studies of the use of phenylephrine to treat faecal incontinence are reviewed. 4 It is concluded that the IAS is a potential target for drug development for the treatment of faecal incontinence.

Investigation of the role of adrenergic and non-nitrergic, non-adrenergic neurotransmission in the sheep isolated internal anal sphincter

Nitric oxide is widely established as an important neurotransmitter in the control of anal sphincter tone; although, a number of other transmitters have also been tentatively implicated. Whilst alpha-adrenoceptor antagonists reduce anal sphincter pressure in man, the role of noradrenaline as a possible transmitter is poorly characterised. We have investigated the contribution of these transmitters to neurogenic relaxations, and evaluated the possible role of a non-nitrergic, non-adrenergic transmitter. The magnitude and duration of neurogenic responses were examined by measuring responses to electrical field stimulation (EFS) in segments of sheep internal anal sphincter following the development of spontaneous myogenic tone. Neurogenic relaxations induced by EFS were significantly reduced in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) suggesting major involvement of nitric oxide as a neurotransmitter. The duration of neurogenic relaxations was inversely related to the frequency of EFS, with contractile responses often manifest at higher frequencies. The duration of relaxations at high frequencies of EFS was increased by bretylium (adrenergic neurone blocker) and prazosin (alpha(1)-adrenoceptor antagonist). At higher frequencies of EFS, 60% of preparations also produced a residual non-nitrergic, non-adrenergic, apamin-sensitive relaxation which was unaffected by vasoactive intestinal polypeptide (VIP) and inhibitors of purinergic responses [suramin, pyridoxal-phosphate-6-azophenyl 2',4' disulfonic acid (PPADS) and alpha,beta-methylene adenosine triphosphate (ATP)]. However, MRS2179 (P2Y(1) receptor antagonist) showed a modest inhibitory effect. We conclude that endogenous noradrenaline acts via postjunctional alpha(1)-adrenoceptors to antagonize neurogenic relaxations that are largely mediated by nitric oxide. Our results indicate the involvement of a non-nitrergic, non-adrenergic, apamin-sensitive transmitter which is inhibited by MRS2179, suggesting a possible role for purines.

Relaxation of the isolated human internal anal sphincter by sildenafil

BACKGROUND

Hypertonicity of the internal anal sphincter (IAS) appears to be involved in the pathogenesis of anal fissure. The relaxant effects of sildenafil, a selective phosphodiesterase 5 (PDE5) inhibitor, on isolated human IAS were investigated.

METHODS

The efficacy (maximal effect, E(max)) and potency (-log IC(50), where IC(50) is half-maximal inhibitory concentration) of the PDE5 inhibitors, sildenafil and zaprinast, and of nitric oxide donors, sodium nitroprusside and glyceryl trinitrate, as relaxants of histamine (0.1 mmol/l)-induced tone were examined in IAS strips under isometric contraction. The presence of PDE5 isoenzymes and changes in intracellular calcium and cyclic nucleotide levels in IAS muscle were tested by real-time reverse transcriptase-polymerase chain reaction, epifluorescence microscopy and enzyme immunoassay respectively.

RESULTS

Sildenafil produced a concentration-related inhibition of the mean(s.e.m.) histamine-induced tone (E(max) 83(2) per cent, - log IC(50) 7.04(0.05); n = 12). Zaprinast produced relaxation to similar degree, but with lower potency. Nitric oxide donors also relaxed IAS. Sildenafil (1 micromol/l) produced a 1.8-fold increase in guanosine 3',5'-cyclic monophosphate content, with no change in adenosine 3',5'-cyclic monophosphate levels. Sildenafil markedly depressed the peak intracellular calcium increase evoked by histamine. PDE5A1, PDE5A2 and PDE5A3 transcripts were expressed in IAS muscle.

CONCLUSION

Sildenafil relaxes the augmented tone of human IAS in vitro. These results support the potential use of this PDE5 inhibitor in the treatment of chronic anal fissure.

Intra-anal application of l-erythro methoxamine gel increases anal resting pressure in patients with incontinence

Background

This study examined the effects of a single dose of intra-anal l-erythro methoxamine on mean anal resting pressure (MARP) and cardiovascular variables in patients with faecal incontinence.

Methods

Patients had anorectal physiology tests and ultrasonography before participating. Six patients received 0·3 and 1 per cent gels on separate days, two patients received 0·3 per cent gel, and two patients received 1 per cent gel. MARP, blood pressure, pulse rate and plasma drug concentrations were measured for 6 h after application.

Results

Intra-anal 0·3 per cent gel caused a rapid, significant rise in MARP lasting 2 h (P = 0·036). In four of these patients, the response was sufficient to increase MARP to within the normal range at 2 h. Application of 1 per cent gel caused a significant rise in MARP for 4 h after application (P = 0·028). There was a significant decrease in pulse at 2 and 1 h respectively after application of 0·3 and 1 per cent gels.

Conclusion

Intra-anal application of l-erythro methoxamine can be used to increase MARP in patients with faecal incontinence. Application of 1 per cent l-erythro methoxamine gel produced a rapid, sustained rise in MARP, which raises the possibility of therapeutic application.

Comparison of the effects of nitric oxide donors and calcium channel blockers on the intrinsic myogenic tone of sheep isolated internal anal sphincter

BACKGROUND

Chronic anal fissure is associated with considerable pain and anal hypertonia. Numerous clinical studies attest to the effectiveness of individual nitro-containing drugs and organic calcium channel blockers in this condition but there are few comparative studies.

METHODS

Isolated segments of sheep internal anal sphincter were prepared for isometric tension recording. The effect of various drugs on myogenic tone was examined in the absence or presence of sodium orthovanadate (SOV), an agent used to mimic anal hypertonia by increasing myogenic tone.

RESULTS

All the drugs tested produced concentration-dependent inhibition of myogenic tone, with the maximum effect ranging from 66.4 per cent (verapamil) to 100 per cent (sodium nitroprusside). Sodium nitroprusside and diltiazem were the most potent, followed by glyceryl trinitrate (GTN), nifedipine and verapamil, which had similar potency, and finally nicorandil. The potency of GTN and diltiazem was reduced threefold in the presence of 1 mmol/l SOV. The combined effect of GTN and diltiazem was greater than the effect of either agent alone, even in the presence of 3 mmol/l SOV.

CONCLUSION

Nitro-containing drugs and organic calcium channel blockers are potent inhibitors of anal sphincter myogenic tone that may be used in combination to treat chronic anal fissure.

L-arginine-induced relaxation of the internal anal sphincter is not mediated by nitric oxide

BACKGROUND

Topical application of L-arginine, the precursor of nitric oxide, reduces resting anal pressure without significant side-effects and may therefore be of benefit in the treatment of anal fissure. This in vitro study investigated the effect of L-arginine on sheep and human isolated internal anal sphincter (IAS) to ascertain the role played by nitric oxide and guanosine 3',5'-cyclic monophosphate.

METHODS

Strips of sheep and human IAS were mounted in isolated organ baths. The effects on myogenic tone of increasing concentrations of L-arginine, D-arginine and other amino acids were evaluated.

RESULTS

L-Arginine, D-arginine and other basic amino acids (L-lysine and L-ornithine) all caused a concentration-dependent reduction in myogenic tone. L-Arginine was the most effective and produced a mean(s.e.m.) maximal reduction in myogenic tone of 78.2(7.1) and 40.2(9.3) per cent in sheep and human tissue respectively. These responses were not affected by N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, or 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase. Changes in pH per se were unable to explain the relaxation fully, but an equiosmolar sodium chloride solution produced a concentration-response relationship similar to that of L-arginine.

CONCLUSION

The ability of L-arginine to reduce myogenic tone is independent of nitric oxide. This effect may be partially pH dependent but the osmolality of the solution appears to be a major factor. Hyperosmolar solutions might be worthy of further investigation as agents that affect anal tone.

Topical L-arginine gel lowers resting anal pressure: possible treatment for anal fissure

PURPOSE

Exogenous nitric oxide donors, such as glyceryl trinitrate, have been used as treatment for anal fissures; however, headaches develop in 60 percent of patients. Nitric oxide produced from the cellular metabolism of L-arginine mediates relaxation of the internal anal sphincter. This study investigated whether topical L-arginine gel reduces maximum anal resting pressure in volunteers.

METHOD

In a two-center study, volunteers received a single topical dose of L-arginine or placebo (Aquagel ). Anal manometry was performed for two hours after application of 400 mg of L-arginine gel or placebo gel to the anal verge in 25 volunteers. Side effects were recorded after single application and also after repeated dosing for three days.

RESULTS

L-Arginine reduced maximum anal resting pressure by 46 percent from a median of 65 cm of water to a minimal value of 35 cm of water ( P< 0.001, Wilcoxon's signed-rank test). The difference between L-arginine and placebo using repeated-measures testing was significant at P< 0.005. No side effects occurred with either gel; in particular, no episodes of headache were recorded.

CONCLUSION

Topical L-arginine gel significantly lowers maximum anal resting pressure; its onset of action is rapid, and duration is at least two hours ( P< 0.01). L-arginine may have therapeutic potential, but further evaluation is needed before it can be used as a possible alternative treatment for chronic anal fissure.

Systemic levels of glyceryl trinitrate following topical application to the anoderm do not correlate with the measured reduction in anal pressure

BACKGROUND

Topical 0.2 per cent glyceryl trinitrate (GTN) lowers resting anal pressure (RAP) and heals two-thirds of chronic anal fissures. Over 50 per cent of patients experience headache, presumably through systemic absorption, but the pharmacokinetics of GTN ointment are unknown. This study evaluated the systemic absorption profile of GTN, and correlation between plasma GTN levels, RAP, haemodynamic variables and side-effects.

METHODS

Thirty healthy volunteers were recruited with local medical school ethics committee approval. Continuous static anal manometry was performed for 10 min before and 2 h after application of 0.2 per cent GTN (0.5 g) to the anoderm. Blood samples were taken from an intravenous cannula, and pulse and blood pressure were measured before application of GTN, and 10, 20, 30, 40, 50, 60, 90, 120 and 180 min thereafter. Details of side-effects were recorded.

RESULTS

GTN was detected in the plasma 10 min to 3 h after topical application. RAP was significantly reduced after 10 min, but had returned to pretreatment values by 120 min. Pulse was statistically unchanged during the study; systolic blood pressure was significantly lower 20-90 min after GTN application, and diastolic pressure was decreased throughout the study. Headaches were experienced by 14 of 30 volunteers after a median (range) of 41 (4-120) min, persisting for 74 (30-176) min, with an intensity score of 19 (5-30) mm represented on a 100-mm linear visual analogue scale. There was no correlation between plasma GTN concentration, RAP, and the onset, duration or intensity of headaches.

CONCLUSION

Topical GTN acts locally on the internal anal sphincter; systemic levels do not contribute significantly to the reduction in RAP. There is marked interindividual variation in plasma GTN levels, and no correlation with haemodynamic variables and side-effects.